Bone remodelling after total hip arthroplasty using

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patients underwent unilateral total hip arthroplasty for primary ... results of 70 patients who underwent total hip ... atrophy, reactive lines, and pedestal formation.
Journal of Orthopaedic Surgery 2006;14(1):32-7

Bone remodelling after total hip arthroplasty using an uncemented anatomic femoral stem: a three-year prospective study using bone densitometry JJ Panisello, L Herrero, A Herrera, V Canales, A Martinez, J Cuenca Department of Orthopedic Surgery, Miguel Servet University Hospital, Zaragoza, Spain

ABSTRACT Purpose. To evaluate the clinical, radiological, and densitometric changes in the bone-remodelling patterns of femoral stems aligned in neutral, valgus, or varus positions. Methods. Between February and October 2000, 70 patients underwent unilateral total hip arthroplasty for primary osteoarthritis using an uncemented Anatomique Benoist Girard (ABG) II stem. 69 patients (30 males and 39 females) with a mean age of 59 years (range, 38–76 years) and a mean body weight of 79.3 kg (range, 29–110 kg) completed 3 years’ followup on bone remodelling. The clinical, radiological, and densitometric changes of the neutral, valgus, and varus groups were evaluated, and the difference in bone-remodelling patterns between the 3 groups was analysed. Results. 54 patients had neutrally placed stems, while varus and valgus malalignment occurred in 6 and 9 patients, respectively. Clinical and radiological

evaluations were very similar among the 3 groups. Only densitometry could detect traceable changes resulting from the differing biomechanics of the neutral, varus, and valgus stem alignments. Conclusion. The ABG II stem design made moderate errors in alignment biomechanically tolerable. Alignment defects had no clinical consequences and resulted in minimal differences in bone remodelling. Key words: arthroplasty, replacement, hip; bone density; durapatite; densitometry; hip prosthesis

INTRODUCTION Bone densitometry (dual energy X-ray absorptiometry, DEXA) has been used to analyse the bone-remodelling pattern associated with different femoral stem implant designs (cemented or uncemented) for the past 15 years. 1–5 Varus or valgus malalignment of the femoral stem has been reported to result in different load transmission patterns than in neutral alignment,6

Address correspondence and reprint requests to: Juan Jose Panisello Sebastia, 3 Maria Espinosa Street, First floor, Door O, 50015 Zaragoza, Spain. Email: [email protected]

Vol. 14 No. 1, April 2006

Bone remodelling after THA using an uncemented femoral stem

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Figure 1 An ABG II stem is scaleshaped in the metaphyseal area, coated with a hydroxyapatite layer, and has a thin short ultrapolished tail.

with poor long-term results.7 Some implant designs have proved to be more clinically and radiologically tolerant of these alignment errors by virtue of the specific geometries, load transmission patterns, and the higher elasticity of the titanium alloys. 8,9 We assessed the clinical, radiological, and densitometric results of 70 patients who underwent total hip arthroplasty (THA) using an uncemented anatomic femoral stem, and analysed the differences in boneremodelling patterns among implants aligned in a neutral position, at >5Ο varus, and at >5Ο valgus.

MATERIALS AND METHODS Between February and October 2000, 70 patients underwent unilateral THA in the Miguel Servet University Hospital using an uncemented anatomic femoral stem (Anatomique Benoist Girard [ABG] II; Stryker Howmedica, Staines, England). Patients were followed up for 3 years on bone remodelling with the ABG II stem. Patients diagnosed with unilateral (the healthy opposite hip was used as a control) primary osteoarthritis (not metaphyseal deformity or severe osteoporosis) and given informed consent were included. The local research ethics committee approved this prospective controlled study. The diagnoses of the patients were hip osteoarthrosis (n=64), avascular necrosis (n=5), and rheumatoid arthritis (n=1). 69 patients (30 males and 39 females) with a mean age of 59 years (range, 38–76 years) and a mean body weight of 79.3 kg (range, 29–110 kg)

Figure 2

Bone densitometry showing the 7 Gruen zones.

completed the 3-year follow-up. The remaining one patient was lost to follow-up who had a neurovascular stroke and became wheelchair-bound 19 months after THA. All patients were implanted with the ABG II stem and uncemented cup (the cup was not part of the study). The ABG II is an uncemented and anatomic stem, with press fit metaphyseal fixation, made of titanium, molybdenum, zirconium, and iron. The stem features a scale-shaped design to increase stability at the metaphyseal level, and is coated with a 70-micronsdeep hydroxyapatite layer. The tail of the implant is short and thin so as to avoid endomedullary contact with the diaphysis, and is ultrapolished to avoid bone fixation at this level (Fig. 1). The ABG II evolved from the earlier ABG I design, which has a longer and more voluminous tail to facilitate neutral positioning. However, the ABG I surface is often susceptible to bone growth and causes proximal bone malfunction. Patients were operated on by 4 different surgeons through the posterolateral approach. The acetabulum was progressively reamed to the cup diameter calculated preoperatively. The cup was inserted by

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Figure 3 Anteroposterior radiographs taken at postoperative 3 years showing (a) a neutrally aligned femoral stem with cortical atrophy in zones 1 and 7 and cortical thickening in zones 2 and 6. (b) A varus-aligned femoral stem with significantly more cortical atrophy in zone 1 than the neural group. (c) A valgus-aligned femoral stem with significantly more cortical atrophy in zone 7 than the neural group.

press fit, usually with 3 pins. After osteotomy of the femoral neck, a piece of cancellous bone was removed using a box chisel to allow direct access to the femoral canal, which was prepared by progressive broaching. With the final broach in place, along with a modular trial neck and head, a trial reduction was performed to check hip stability. The trial insert was removed, the cup interior was cleaned and dried, and the definitive polyethylene layer and stem were placed. The capsule was sutured, enclosing the femoral head, and the short rotator muscles were attached to the greater trochanter with transosseous sutures. Fascia lata, subcutaneous tissues, and skin were carefully sutured. Patients were clinically evaluated according to the Merle d’Aubigne and Postel (MAP) score 10 measuring pain, joint mobility, and ability to walk. Varus and valgus angulation (the difference between the femoral diaphyseal axis and the stem’s main axis) were measured and a difference of >5Ο was considered varus or valgus. The 7 Gruen zones were inspected for the presence of radiolucent lines, osteolysis, cancellous condensation, cortical hypertrophy or atrophy, reactive lines, and pedestal formation according to the criteria by Engh et al.11 Bone stock variation was measured in each of the 7 Gruen zones in a 30x30–pixel square using a Hologic QDR 1000 densitometer (Hologic, Waltham [MA], US) [Fig. 2].

Patients were placed in a supine position on the scanner table, with knees and hips in full extension. The whole limb was in a neutral position supported by a rigid plastic device fastened by Velcro tape to guarantee measurement reliability. The preoperative and postoperative first-year and third-year MAP scores, radiographs, and bone stock densitometric measurements among the neutral, varus, and valgus groups were compared using parametric or nonparametric tests, depending on the number of patients and whether the samples were independent or related. Chi squared test was used with Pearson correlation for qualitative variables. A p value of